Conventionally, there has been known a nonaqueous electrolyte secondary battery provided with a wound-type electrode assembly (see JP-A-2013-219027). To be more specific, the nonaqueous electrolyte secondary battery includes: an electrode assembly formed by winding an electrode having positive polarity (positive electrode), an electrode having negative polarity (negative electrode), and a separator; a battery container which houses the electrode assembly together with an electrolyte solution; a lid which is a lid for closing an opening of the battery container and in which an electrolyte solution pouring port is formed; and a sealing plug which seals the electrolyte solution pouring port. In the electrode assembly, strip-like electrodes and strip-like separator are wound around in an alternately layered state.
In manufacturing steps of the above-mentioned nonaqueous electrolyte secondary battery, the opening formed in the battery container in which the electrode assembly is stored is closed by the lid, the electrolyte solution is poured in the battery container through the electrolyte solution pouring port formed in the lid and, thereafter, the electrolyte solution pouring port is sealed by the sealing plug. In the battery container, the poured electrolyte solution impregnates (infiltrates) into the electrode assembly. The impregnation of the electrolyte solution is specifically described below.
The electrode assembly is formed by winding the electrodes and the separator in a layered state. Accordingly, the electrolyte solution poured in the battery container impregnates into spaces formed between the electrodes and the separator wound in a layered state from an end portion of the electrode assembly in the winding axis direction. In this case, the electrolyte solution infiltrates into the inside of the electrode assembly from both ends of the electrode assembly in the winding axis direction respectively and hence, a part of a gas such as air present in a space between the electrode and the separator may not be discharged from the space between the electrode and the separator (that is, a part of the gas may be confined in a center portion of the electrode assembly by the electrolyte solution which impregnates into the inside of the electrode assembly from both end of the electrode assembly), and may be left at the center portion of the electrode assembly in the winding axis direction.
In this case, the electrolyte solution may not impregnate into the inside of the whole electrode assembly uniformly and hence, a formed state of a negative electrode protection film (solid electrolyte interface: SEI) in the inside of the electrode assembly may become non-uniform. In a portion of the electrode assembly where the formation of the negative electrode protection film is insufficient, charge-discharge is repeated in the nonaqueous electrolyte secondary battery and hence, an irreversible reaction is liable to progress in such a portion compared with other portions.